1. Field of the Invention
The present invention relates to a motor control apparatus, and in particular, to a motor control apparatus suitable for a recording system for recording video signals on a rotary recording medium in an electronic still camera. An object is shot by use of an imaging device such as a solid-state imaging device to record the video signals produced by the imaging device on a rotary recording medium such as a magnetic disk. Also, the motor control apparatus can be used in a video signal recording system where video signals are read from a video memory and recorded on a rotary recording medium.
2. Description of the Prior Art
A conventional electronic still camera uses an imaging device such as a solid-state imaging device or an imaging tube in combination with a recording system using as a recording medium, a low-priced rotary recording medium such as a magnetic disk having a relatively large storage capacity, to shoot an object electronically as a still picture and to record the object on the rotating magnetic recording medium for reproducing the picture by using a separate television system or a printer.
In such a rotating recording medium, a favorable recording is only achieved when the rotating recording medium is in a predetermined stationary rotation. The conventional electronic still camera systems which have been put to the practical use include a still camera system constructed with a magnetic disk having a phase generator, PG, provided on a core of the magnetic disk so that a phase pulse is sensed in response to each rotation of the magnetic disk. The phase pulse is used for various control operations, for example, in a still camera system, the rotation servo of the magnetic disk, namely, the rotating speed and phase, are controlled by use of the phase pulse.
To reproduce a well-synchronized picture from the magnetic disk soon after the tracking operation of a magnetic disk in a playback system, it is advantageous that the initial position for recording the field of video signals on a track, namely, the position to record the vertical sync signal is within a predetermined range relative to the position where a PG signal is sensed. According to the standards of the ordinary recording format applied to the electronic still camera, the range is specified to be within 7H.+-.2H of the position of PG generation, where H represents a period of time corresponding to one horizontal scanning line.
Consequently, in a case where video signals are supplied from a signal source, such as an imaging device, that has controllable timing for reading out, and the video signals are to be recorded on a magnetic disk at a preset timing, the recording timing of a field or frame of video signals can be controlled to include the vertical sync signal within the specified range relative to the position of the PG generation as described above.
In an electronic still camera, the first stroke of the shutter release operation enables a motor to start driving a magnetic disk and the PLL servo is accomplished in response to an FG signal followed by a PG signal, so that when the rotation of the motor reaches a predetermined rotating speed and phase with a predetermined tolerance range, video signals are read from the imaging device in synchronism with the PG signal to be recorded on the magnetic disk. However, contact of the sheet surfaces of the magnetic disk with the magnetic head and the regulating plate causes mechanical unstability, and hence, the load applied to the driving motor is variable. As a result, even if such a PLL servo is set up, the system cannot rapidly respond to the variation in the load. Moreover, although the servo system is using automatic frequency control, AFC, and the automatic phase control, APC, as described above, it is sensitive to the influence from the environmental temperature of the system. It has also been required that the stationary rotation of the magnetic disk, obtained by applying the PLL servo under such a temperature change, be within the predetermined allowable range.